1. Field of the Invention
This invention generally relates to an interface circuit for digital signal transmission or communication between separate devices, and specifically relates to an interface circuit for digital sound or video signal transmission between separate devices in an audio system or a video system.
2. Description of the Prior Art
Advanced audio or video systems handle sound or display data in the form of digital signals at certain stages. In these systems, the digital data signals are finally converted into corresponding analog data signals. The deterioration in quality of such a reproduced analog data signal mainly results from the following two causes:
(1) A failure in accurate transmission and digital-to-analog (D/A) conversion of a digital data signal;
(2) Contamination of an analog data signal caused by the ingress of noises which are contained in a digital data signal or which are generated in a digital signal processing unit.
One known way of removing the previously-mentioned cause (2) of the signal deterioration is to dispose a digital signal processing unit and a D/A converting and analog signal processing unit in separate chassis respectively to reduce or prevent interference between the two units. In some cases, these two units in the separate chassis are housed in a common cabinet. In other cases, they are housed in separate cabinets respectively.
In digital audio systems, another known way of removing the previously-mentioned cause (2) of the signal deterioration is to use a photocoupler in an interface circuit performing digital signal transmission from a digital signal processing unit (a signal transmitting unit) to a D/A converting and analog signal processing unit (a signal receiving unit). In this interface circuit, a digital signal is transmitted between the two units via the photocoupler. The photocoupler enables electrical isolation between the two units while maintaining the signal transmission between them. According to this electrical isolation, the two units are generally activated by separate power supplies and grounds of the two units are separate. The electrical isolation is advantageous in that if a ground current of the signal transmitting unit is varied by a change of a condition such as a change of a load of the related power supply, the variation in the ground current of the signal transmitting unit will not be transmitted to the signal receiving unit. Furthermore, the electrical isolation including the ground isolation is usually effective in cutting off common mode noises.
Generally, such a known photocoupler-based interface circuit is useless in removing the previously-mentioned cause (1) of the signal deterioration. Specifically, in this known interface circuit, once a digital sound signal has unwanted components such as jitters, noises, or ripples, a corresponding analog sound signal will be deteriorated by these unwanted components. The deteriorated analog sound signal lowers tone quality and fidelity in sound reproduction.